This tart just may be the perfect antidote for a dreary winter’s day. You start with a hazelnut cookie crust strewn with grapefruit zest, followed by a thin layer of bittersweet chocolate, rounds of ruby-red grapefruit perched atop a billowy hazelnut cream, and a light glaze of citrus marmalade over it all. Any citrus would work here, but the sour notes of the grapefruit pair perfectly with the dark chocolate and sweet hazelnut filling.
A few notes about simple techniques that make a big difference here: to make the hazelnut meal for the tart crust and the cream filling, freeze your toasted hazelnuts for at least 15 minutes, then combine them with the powdered sugar to produce a finely ground hazelnut meal. Also, dry the grapefruit segments between layers of paper towels for an hour or so prior to assembling the tart, to prevent the release of excess juice during baking.
Almonds are a good substitute for hazelnuts in the tart crust. And while the flavor of toasted nuts is hard to beat, your tart won't suffer if you skip this step because you're short on time.
To toast hazelnuts: Place hazelnuts in single layer on rimmed cookie sheet. Toast for 8 to 10 minutes in a 350 degree F oven, shaking the pan once or twice to ensure even browning. Once cooled, put toasted hazelnuts in the freezer for 15 minutes or longer.
To grind hazelnuts: Combine frozen hazelnuts with powdered sugar in bowl of food processor. Pulse -- stopping every few seconds -- until the hazelnuts are uniformly and finely ground, about 15 to 20 pulses.
Preheat oven to 350 degrees F.
Prepare tart crust: Put flour, hazelnut/powdered sugar mixture, salt, and grapefruit zest in the bowl of a food processor and pulse a few times to combine. Add the butter and pulse until the butter is coarsely cut in, about the size of peas -- about 5 to 7 times. Stir the yolk to break it up, pour over the dry ingredients, and pulse (about 5 seconds each time) until the dough starts to clump and come together. If some of the dough is still dry and crumbly, that’s okay, since it’s a press-in crust. Put the dough into a 9-inch tart pan, pressing the dough evenly over the bottom and up the sides of the pan. Prick the dough all over with a fork, and freeze for 30 minutes or longer.
Bake the tart crust: Bake the crust for about 25 to 30 minutes, or until the crust is golden brown. Keep a close eye on the crust to make sure it doesn’t burn -- you can always lay a sheet of foil across the top if it’s browning too quickly. About half-way through the baking time, check the dough to see if it has puffed up; if so, press it down with the back of a spoon so you’ll have a deep enough shell for the filling. Remove from oven, and cool completely.
Prepare grapefruit: Slice off the top and bottom, then with a curving cut, remove the peel, working in sections. Slice the grapefruit into ¼-inch rounds and remove seeds. Dry the grapefruit rounds between several layers of paper towels -- blotting the fruit to remove excess moisture -- for at least 30 minutes but preferably longer.
Prepare hazelnut cream: Put the butter and sugar in a mixer fitted with the paddle attachment and combine until the mixture is smooth. Add the hazelnut/powdered sugar mixture, flour, and grapefruit zest, and mix -- then add the egg and vanilla. (You can also do this step by hand with a rubber spatula.) Mix until well combined -- about 15 to 20 seconds.
Prepare chocolate: Melt, using either double-boiler or microwave; stir until smooth.
Assemble tart: Spread the melted chocolate in an even layer over bottom of tart shell. Freeze the tart shell for about 15 minutes until the chocolate has fully solidified. Next, pour the hazelnut cream over the chocolate layer, smoothing the top. Arrange grapefruit rounds over the top.
Bake the tart: Bake for 50 to 60 minutes, or until the cream has turned golden brown. A knife slipped into the center of the cream should come out clean when it’s done. Remove from oven and cool completely.
Glaze the tart: Right before serving, heat the marmalade and 1 teaspoon water for about one minute in microwave, or until bubbling. Brush the glaze over the entire surface of the tart.